High voltage pulse generating apparatus

ABSTRACT

A combustion control device for controlling the combustion of a liquid fuel such as petroleum fuel. The device incorporates a capacitor-discharging ignition, a prepurging operation, a postignition extending operation and safety means for preventing accidents which may occur due to system faults.

United States Patent 91 Ishida Mar. 5, 1974 HIGH VOLTAGE PULSE GENERATING APPARATUS [75] lnventor: Yoshio Ishida, Osaka, Japan [73] Assignee: Diamond Electric Mfg. Co. Ltd.,

Osaka, Japan [22] Filed: July 25, 1972 [21] Appl. No.: 275,006

[52] U.S. Cl 317/96, 317/151, 431/29 [51 1 Int. Cl. F23q 3/00 [58] Field 01 Search... 317/96, DIG. 8, 15]; 431/29,

[56] References Cited UNITED STATES PATENTS 3,384,439 5/1968 Wolbridge 317/96 3,287,608 11/1966 Pokrant 317/DlG. 8

Primary Examiner-Gerald Goldberg Assistant Examinerllarry E. Moose, Jr.

Attorney, Agent, or Firm-Sughrue, Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT A combustion control device for controlling the combustion of a liquid fuel such as petroleum fuel. The de vice incorporates a capacitor-discharging ignition, a prepurging operation, a post-ignition extending operation and safety means for preventing accidents which may occur due to system faults.

7 Claims, 5 Drawing Figures PATENTED HAR 51974 FIG] HIGH VOLTAGE PULSE GENERATING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a combustion control device for use with a burner, and more particularly to a control device incorporating a prepurge operation and a safety monitoring circuit.

2 Description of the Prior Art Heretofore, in an electric ignition system for a burner utilizing sprayed fuel, a commercial AC power supply, such as 60 Hz, 1 15 volts connected by a step-up transformer such as 7KV secondary output, is used. Several electromagnetic relays such as a fuel valve operation relay, and safety means such as a temperature switch are also used in the system. The systems are complicated, and are located in high temperature environments with the result that the endurable life is short and their reliability is low.

Therefore, it is an object of the present invention to provide a highly reliable fuel control device which is simple in construction and compact because of solidstate components.

SUMMARY OF THE INVENTION The combustion control device of the present invention comprises means for controlling the fuel supply and a transformer for supplying ignition energy to the fuel. The device further includes means for detecting a flame to confirm the ignition of the fuel, means for determining the continuation of the operation during a predetermined time, means for detecting the continuation of the operation beyond a predetermined time, and means for rapidly discharging a capacitor through the transformer.

Other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of the various devices combined with one embodiment of the fuel control device of this invention.

FIG. 2 is a block diagram of a fuel control device of another embodiment of the present invention.

FIG. 3 shows the details of the relay energizing control circuit of the relay in FIG. 2.

FIGS. 4a and 4b are side sectional views of the adjustable safety switch utilized for the fuel control device of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, a plurality of terminals T, to T are provided in the control device surround by the broken line. The terminal T, is connected through the primary winding of an ignition transformer IG to one of the terminals of AC power supply, the terminal T is directly connected to the AC power supply, the terminal T is connected to a flame detector F such as a CdS element, and the terminal T,, is connected to a fuel valve operation solenoid valve V. The terminal T is connected to a warning device A and the terminal T is connected to a blower and hydraulic pump drive motor M. The terminal T is connected through a normally closed contact Th of a thermostat to the terminal T,,. The terminal T is connected to the other terminal of the AC power supply. A spark discharge gap P for fuel ignition is connected to the secondary winding of the transformer IG. The terminal T is connected through the normally open contact SS2 of a safety switch SS to the terminal T The terminal T is connected through a normally closed contact of the safety switch S5 to the terminal T The terminal T is connected through a series circuit of a resistor r energizing coil R, of a first electromagnetic relay and diode D to the terminal T The resistor r, and the energizing coil R, are shunted by a smoothing capacitor C The terminal T is connected through a heater resistor TSH of a thermal switch TS and the normally closed contact R", of the first electromagnetic relay to the terminal T The terminal T is connected through a series circuit of diode D energizing coil R of the second electromagnetic relay, the normally closed contact a of the switching contact R, of the first electromagnetic relay, the heater resistor SSH of the safety switch SS, and the normally open contact T81 of the thermal switch TS to the terminal T The energizing coil R is shunted by the smoothing capacitor C The normally open contact b of the switching contact R, is connected through a resistor r, to the terminal T,,. The terminal T is connected through the normally open contact R' of the second electromagnetic relay to the terminal T The terminal T is further connected through the series connection of the resistors r, and r to the contact T51 of the thermal switch. The terminal T, is connected to a main capacitor C, and one connecting point c of the resistor r in parallel therewith. The main capacitorC, and the other connecting point-d of the resistor r are common with one connecting point of the diode D, connected in opposite parallel with a thyristor S. The thyristor S and the other connecting point e of the diode D, are connected to the connecting point T81 of the thermal switch. The gate electrode of the thyristor S is connected to the connecting point f of the resistors r, and

r Further, protecting diode D is connected in parallel with the resistor r Then, the operation of thus constructed control device of this invention will now be described in various cases.

At first, as normal operation, when a main switch (not shown) is closed, a line voltage is applied between the terminals T and T Since the contacts Th of the thermostat and the contacts SS1 of the safety switch are closed, the blower and an electric motor M for driving a hydraulic pump start the prepurging of the combustion chamber of a furnace, and the build-up of the fuel hydraulic pressure is started. Separately, after a predetermined time such as 20 seconds, the normally open contact T81 is closed by the energization of the heater TSH of the thermal switch and heating thereof. The predetermined time is selected to complete the buildup of the hydraulic pressure and the prepurging.

the resistors r, and r,, and applied to the gate electrode of the thyristor S, so that the thyristor S is turned on. It is preferred that the phase angle of the voltage wave at this time is approximately 60 to select the values of the resistors r, and r Immediately before the thyristor S is turned ON, the voltage corresponding to the charge stored in the capacitor C, and the line voltage is applied between the main electrodes of the thyristor S, and as the thyristor S is turned ON, the raised voltage is applied to the primary winding of the transformer. Therefore, in the iron core of the transformer, the magnetic flux due to the discharge of the capacitor is added to the flux corresponding to the accumulated energy and, accordingly, a voltage, raised by far greater coefficient than the ratio of the secondary winding to primary winding, is induced in the secondary coil so that sparking is generated at the gap P.

The coil R of the second electromagnetic relay is energized by the current flowing from the heater SSH of the safety switch so that the contact R is closed, and a change in the strain on the thermal element (not shown) is started by the generation of the heat of the heater SSH. The fuel supply means control solenoid V is energized by the closing of the contact R and the fuel valve (not shown) is opened. Thus the sprayed fuel is ignited by the spark in the combustion chamber of the furnace.

When the flame of the combusion of the ignited fuel grows sufficiently, it is detected by the flame detector F. The flame detector is composed of photoconductive material such as CdS, and when it detects the flame, its electric resistance is abruptly lowered. Therefore, upon the detection of the flame the current flowing through coil R, is increased, and the coil R, is energized, so that the contact R, is switched to the side b whereby the contact R is held closed. At the same time, the contact R, is opened. The current flowing to the heater SSH of the safety switch is also cut off before the safety switch is operated. The heater TSH of the thermal switch starts cooling upon the opening of the contact R",, and after a short time the contact T81 is opened, thus resetting itself. Therefore, in a short period of time after the flame has sufficiently grown, that is after proper post-ignition, the ignition circuit composed of the thyristor S, main capacitor C, and transformer IG return to an inoperative state. In such a state, the contact R',-b is closed. R is also closed. R, is open, and the other components are as shown in FIG. 1. The motor Mremains operating and the fuel valve remains open, thus continuing the normal operation.

When the water temperature or room temperature reach a predetermined level as the burner continues in normal combustion, this is detected by the thermostat, so that the contact Th is opened. Thus, the current flowing through motor M and coils R, and R is cut off so that the combustion is stopped with the result that the system returns to the original state. When the contact Th is reclosed by the lowering of the water temperature, all above steps are repeated, and control by the thermostat is repeated.

During the above normal operation, when the flame is extinguished for any reason, the resistance of the flame detector F is increased, the current flowing through the coil R, is reduced, thus deenergizing the relay, so that the contacts R, and R, are returned to the position shown in the drawing. Since the contact T51 is opened as shown, the coil R is also deenergized,

and the contact R is reset, and the fuel supply is shut off. However, since the line voltage is applied, and the contacts Th and SS1 are closed, the operation of the motor M is continued, thus purging the unburned residual fuel. The contact T81 is closed at a predetermined time by the reenergization of the thermal switch TSH, and similar to the above, reignition is tried again. If there is no trouble, it returns to the normal operation. Heat is accumulated at a constant rate by the current flowing through the heater SSH and contacts R',-a and coil R This heat after a predetermined time operates the safety switch SS, opening the contact SS1, and closing the contact SS2. When sufficient ignition of fuel and flame growth do not occur during the predetermined time, or even if the flame grows but it is not detected, or when complete function of the electromagnetic relay contact R, is lost, the contact SS1 is opened, the operation of motor M is stopped, and the contact SS2 is closed, operating the warning device A. This safety switch is a locking device, and after the check of the oil feed system and the other components and removal of the abnormal condition, it is preferred to release the locking device manually.

Then, another embodiment shown in FIG. 2 of this invention will now be described in detail.

The device shown in FIG. 2 is basically the same as that shown in FIG. 1 except for the flame detecting circuit, and the third electromagnetic relay corresponding to the thermal switch for adjusting the sensitivity and response time shown in FIG. 1. In the device shown in FIG. 2, in order to prevent the abrupt voltage change presented when the thyristor is energized, only the energy of a half wave of the line voltage is used.

The various components connected to the terminals T, to T outside of the control device are similar to those shown in FIG. 1; therefore, the description is omitted in FIG. 2. The primary winding of the stepdown transformer Tr is connected between the terminals T and T The secondary winding of the transformer Tr is connected through the diodes D and D to a node k. The neutral point of the secondary winding is connected to the terminal T The series connection of the main capacitor C,, diode D and normally open contact T, of the third electromagnetic relay is connected between the terminals T, and T The path between the main electrodes of the thyristor S is connected between the connecting point g of the main capacitor C, and the diode D and the terminal T Further, the series connection of the resistor r resistor r and zener diode ZD is connected between the terminal T and the secondary winding of the transformer Tr. The connecting point H of the resistors r, and r, is connected to the gate electrode of the thyristor S. The diode D, is in an opposite parallel with the path between the main electrodes of the thyristor S. The protecting diode D is provided between the gate electrode of the thyristor S and the terminal T The main capacitor C, is shunted with the resistor r,,.

The series connection of the energizing coil R of the second electromagnetic relay, normally closed contacts R',-a of the first electromagnetic relay, diode D and the heater resistor SSH of the safety switch SS is connected between the connecting point i of the diode D and the normally open contact T, and the terminal'T The energizing coil R is shunted by the smoothing capacitor C The terminal T, is connected through the normally open contact R, of the second electromagnetic relay to the terminal T The terminal T is connected through the normally open contact SS2 of the safety switch to the terminal T The terminal T is connected through the normally closed contact SS1 of the safety switch to the terminal T The flame detecting circuit and the third electromagnetic relay control circuit u, which will hereinafter be described, are connected to the terminals T and T the connecting point k, and the normally open contact b of the first electromagnetic relay.

In the flame detecting and third electromagnetic relay control circuit shown in FIG. 3, the connecting point k is connected through the resistor r, to the normally open contact b. In addition, the connecting point k is connectedthrough the capacitor C to the terminal T The terminal T is connected through the capacitor C to the terminal T The terminal T is connected through the resistor r to the connecting point k, and is also connected to the base of the transistor Q The emitter of the transistor O is connected through the resistor r to the terminal T The collector of the transistor Q, is connected through the resistor r, to the connecting point k. The collector of the transistor 0, is connected through the resistor r to the base of the transistor Q and the base is connected through the resistor r to the terminal T The emitter of the transistor 0 is connected to the emitter of the transistor Q and the collector of the transistor O is connected through the energizing coil R of the electro-magnetic relay to the connecting point k. The collector of the transistor O is connected through the diode D to the connecting point k, and further through the resistor r to the base of the transistor O Therefore, the transistors Q and 0-,, form an emitter-coupled binary or Schmitt trigger circuit together with the cooperative components. The emitter of the transistor O is connected to the terminal T and the collector thereof is connected through the resistor r to the connecting point k. The collector of the transistor Q; is also connected through the diode D to the base of the transistor Q The collectoremitter-path of the transistor O is shunted by the timing capacitor C The emitter of the transistor Q 4 is connected to the terminal T The collector of the transistor O is connected through the resistor r to the connecting point k and to the base of the transistor Q The emitter-collectonpath of the transistor 0., is shunted by the timing capacitor C The emitter of the transistor O is connected to the terminal T The collector of the transistor O is connected through the resistor r to the connecting point k, and to the base of the transistor Q The emitter-collector path of the transistor O is shunted by the resistor r The emitter of the transistor O is connected through the resistor r to the terminal T and the collector thereof is connected through the resistor r to the connecting point k. The collector of the transistor 0 is connected through the resistor r to the base of the transistor Q The base of the transistor O is connected through the resistor r to the terminal T and the emitter thereof is connected to the emitter of the transistor Q The collector of the transistor 0 is connected through the energizing coil T, of the third electromagnetic relay to the connecting point k. The diode D is provided in parallel with the coil T Therefore, second emittercoupled binary or Schmitt trigger circuit is formed by the transistors Q and O and related components thereto. Zener diode ZD is connected between the output terminal of the transformer Tr and the resistor r Diode D is connected in series with the main capacitor C The operation of the device shown in FIGS. 2 and 3 will now be described. When the line voltage is applied between the terminals T and T current flows through the normally closed contacts SS1 and Th between the terminals T and T whereby the motor M (FIG. 1) is energized to prepurge the system as previously described. At the same time, the capacitor C (FIG. 3) is charged by the output Voltage of the transformer Tr. The transistors O O and Q, are made conductive by the increase in the voltage across capacitor C When the voltage across capacitor C reaches a predetermined value at a predetermined time such as about 20 seconds, the transistor Q turns ON, transistor 0., turns OFF, and the transistor 0 turns ON. When the transistor O is ON, the coil T; of the relay is energized so that the normally open contact T, (FIG. 2) is closed. This corresponds to the closing of the contact TSl of FIG. 1. Thus, when the terminal T is positive, the current flows through the diode D and charges the main capacitor C and accumulates magnetic energy in the ignition transformer IG (FIG. 1). When the terminal T is positive, the voltage applied to the gate of the thyristor S reaches a predetermined value, whereupon the thyristor S is made conductive, so that the charge stored in the-main capacitor C is discharged through the thyristor S, terminal T ignition transformer IG and terminal T Therefore, both the electric energy accumulated at the main capacitor C and the magnetic energy accumulated at the transformer IG are discharged by spark plug P. I

The coil R of the relay is energized by the current flowing through the heater resistor SSH of the safety switch, diode D and normally open contacts R' -a. Thus, the normally open contact R' is closed, the fuel solenoid V (FIG. I) is energized, the fuel valve is opened, and the fuel supply is started. As previously described, the spark discharge occurs at the spark plug P, and accordingly the fuel is ignited. As a result of the growth of the flame, flame detector F (FIG. I) detects the flame, and if the resistance of the detector is reduced, the transistor Q having a base connected to the flame detector F through the terminal T turns OFF, and accordingly the transistor turns ON. Thus, the coil R of the relay is energized by the current flowing through the transistor Q and the contact R is switched to the contact b so that the heater resistor SSH of the safety switch is cut out. At the same time, the transistor O turns OFF so that capacitor C starts to change with the current flowing through the resistor r As the capacitor C is charged, the voltage across the capacitor C reaches a predetermined value after a predetermined time such as 15 seconds, and the transistor Q turns ON. Thus, the charge stored in the capacitor C is discharged through the transistor 0,, with the result that the transistor O turns OFF, while the transistor 0 turns ON, the transistor Q turns OFF, and the coil T I of the relay is deenergized. Thus, the contact T, is opened, and the function of the ignition system including the thyristor S is stopped. Thereafter the burner device is operated by the normal combustion process.

After the fuel supply and the spark discharge are started, if sufficient growth of the flame is not obtained in a predetermined time, the flame detector F does not operate, and accordingly the coil R is not energized so that the connecting point R is not switched. Accordingly, the energization of the safety switch to the heater resistor SSH is continued. If the energization of SSH exceeds a predetermined time limit, the safety switch is operated, the normally closed contact SS1 is opened, and the normally open contact SS2 is closed. Thus, as described in FIG. 1, the warning device A is operated at the same time the other functions of the system are all stopped. Then, before the start of the device, if the flame detector F is operated, the transistor Q turns ON by the application of the line voltage to the terminals T and T the coil R is energized, and the contact R is switched to the contact b. By selecting a suitable value for the resistor r (FIG. 3), the current supplied to the coil R through the resistor r, is insufficient to close the normally open contact'R but is sufficient to hold the closed contact R in closed state. The fuel valve is not opened.

The other functions are the same as those in the embodiment shown in FIG. 1.

FIG. 4 shows the structure of the safety switch for use in the combustion control device of this invention. As shown in FIG. 4a, the safety switch SS comprises a housing 1, a bottom plate 2, a microswitch 3 contained therein, and switch operating mechanism. The microswitch 3 is conventional and is opened and closed through a push-button 4. The operating mechanism has a bracket 6 fixed onto the'bottom plate 2 by a proper means, and tab-portions 6a, 6b and 6c are bent at right angles integral with the bracket 6. An operating arm '7 is provided at the tab portion 6a through a U shape leaf spring 8. The one end of the arm 7 is secured to the spring 8 by a proper means such as rivets through end 7a which is bent at a right angle. The other end of the arm 7 forms a substantially Z shape to project out of the housing 11 through the hole 10. At right angles to the arm 7 is a bimetal piece 9 attached at end 9a. The portion between 9b-9c at the free end of the bimetal piece 9 is bent as shown in the drawing to be substantially J in shape. Second leaf spring 10 is provided in the, operating mechanism. The one end 10a of the spring 10 is secured to the second tab portion 6b of the bracket 6 by means such as rivets, and the other end 10b of the spring 10 is bent generally in a Z shape. The spring 10 is normally engaged with the end 90 of the bimetal 9 at the end 10b, that is, the end lltlb is pushed up from the bottom to be biased as shown in FIG. 4a. Another leaf spring is fixed to the microswitch l at one end between the spring and the pushbutton 4 so that the free end of the spring 5 is always engaged with the bottom surface of the spring 10. The tab portion 6c of the bracket 6 is a stopper for restricting the movement of the spring 5. The heater resistor 12 (corresponding to SSH in FIG. 1) of the safety switch is surrounded by the U shape portion near the end of the bimetal piece 9. The end ofa screw ll 1 passes through the hole (not shown) provided at the tab portion 6a of the bracket 6 and both legs of the spring 8 are screwed into the bent portion 7a of the spring 7.

Then, the operation of the safety switch thus constructed will now be described. At first shown in FIG. 4a, the end 10b of the spring 10 is engaged with the end 90 of the bimetal 9, and when the free end of the spring 5 and the push-button 4 is raised, the normal contact of the microswitch 3 is closed, while the back-contact is open. It is assumed that the bimetal 9 is strained in the direction that the bending of the ends 9b-9c is increased as the temperature rises. The current flows through the heater resistor 12, and as the resistor 12 generates heat, the temperature of the bimetal 9 is increased so that the end 9c is moved gradually to the right in FIG. 4a. At a predetermined temperature, the end 96 is disengaged from the end 10b of the spring 10. Thus, the end 10b of the spring 10 is lowered as shown in FIG. 4b, and pushes down the push-button 4 through the spring 5. As a result, the normal contact of the microswitch is opened, while the back-contact is closed. Simultaneously, the end 10b of the spring 10 is engaged with the side end of the bimetal 9 to hold the relation shown in FIG. 4b. Therefore, when the current to the heater resistor is cut off so that the temperature of the bimetal 9 is lowered, even if the lower end 9b-9c of the bimetal is returned to the state shown in FIG. 4a, the springs 10, 5 and push-button 4 do not return to the state shown in FIG. 4a. That is, the microswitch 3 is held in the state that the normal contact is open, that is, it is locked.

After the end 9b-9c of the bimetal 9 is returned to the state shown in FIG. 4a, and after the end 7b of the arm 7 is pushed down manually, the mechanism is reset when released by the springs 8 and 5. The end 9c of the bimetal 9 engages the bottom side of the end 10b of the spring ill), and further pushes up the end lltib to return to the state shown in FIG. 4a. Thus, the spring 5 and the push-button 4 return to the original state, so that the microswitch 3 is reset.

The end 7a moves leftwardly or rightwardly in FIG.

4a by rotating the screw 11 and the gap g between the legs may be varied. Thereby, the bimetal piece 9 integral with the arm 7 moves leftwardly or rightwardly as a whole, and accordingly the end of the bimetal piece 9 moves relative to the end 10b of the spring 10 so that the amount of engagement varies. As the temperature of the bimetal piece 9 rises, the strain amount of the end 9b-9c of the bimetal piece 9 required for disengaging the end 9c also varies. This means that the time limit of the safety switch which disengages end 90 also varies. In other words, by the rotation of the screw 1 1, the predetermined time limit of the operation of the switch SS is easily adjusted. In addition, in operation, since it has no mechanical sliding portions, the set conditions do not become improper thus making the switch reliable and lockable.

- Further, when trouble occurs in the device of this invention, it always fails safe. For example, before the combustion device starts, if the flame detector F senses a light, that is, the resistance is in fact zero, and the main switch (not shown) is thrown in, and the line voltage is applied between the terminal T and T the contact R is opened by the energization of the coil R of the first electromagnetic relay, and accordingly the contact T51 of the thermal switch is not closed. Therefore, the operation of the fuel valve and the operation of the ignition circuit do not occur. If the breakage of line takes place at the heater resistor SSl-I of the safety switch before it moves to the normal combustion state, the coil R of the second electromagnetic relay is not energized or deenergized. Thus, the solenoid V for controlling the fuel supply means is not energized.

While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

I claim: 1. A device for controlling the ignition and combustion of a burner said device comprising a. spark gas means for providing a spark to ignite said burner;

b. ignition transformer means for coupling energy to said spark gap means;

c. main capacitor means for discharging energy stored therein through said ignition transformer means;

(1. normally non-conducting thyristor means connected in series with said main capacitor means and said ignition transformer means to form an ignition circuit for providing the discharge path of said main capacitor means through said ignition transformer means;

e. first timer means connected in series with said main capacitor means for completing said ignition circuit and rendering said thyristor means conducting after a first predetermined time period;

f. flame detector means responsive to a predetermined post-ignition burner flame for interrupting said ignition circuit and rendering said thyristor means non-conducting; and

g. safety switch means connected in series with said flame detector means and said first timer means for timing a second predetermined time period after said first predetermined time period and turning off said burner after said second predetermined time period, wherein said flame detector means prevents said safety switch means from completing said second time period upon the detection of a flame whereby said safety switch means turns off said burner only when said burner is in an operating state and no flame is detected.

2. The device as set forth in claim 1 further including means for purging said burner wherein said burner is prepurged at the beginning of said first predetermined time period.

3. The device as set forth in claim 1 wherein said safety switch means includes a heater and a heat sensitive switch wherein the detection of a flame by said flame detector means cuts the current through said heater thus preventing it from operating said heat sensitive switch.

4. The device as set forth in claim 1 wherein said first timer means includes a thermal switch comprising a heater and a normally open switch.

5. The device as set forth in claim 1 wherein said first timer means comprises a. first Schmitt trigger means in series with said flame detector means;

b. relay means having a contact in series with said main capacitor means;

c. second Schmitt trigger means for energizing said relay means; and

d. timing capacitor means for switching said second Schmitt trigger means after said first predetermined time period, thus energizing said relay.

6. The device as set forth in claim 4 wherein said I safety switch means comprises a. a microswitch; b. a first leaf spring for switching said microswitch; c. a stationary bracket; (1. a U shaped leaf spring mounted on said bracket,

the open end of said U being adjustable; e. an arm mounted on one arm of said U shaped leaf spring; and f. a J shaped bimetal piece mounted on said arm and positioned to hold said first leaf spring in a position wherein said microswitch is in a first state whereby upon the heating of said bimetal piece to a predetermined level, said bimetal piece moves to a position to release said first leaf spring thus switching said microswitch to a second state, wherein the time required to reach said predetermined level is said second time period. 7 7. The device as set forth in claim 6 said safety switch means further including means for adjusting the opening of said U shaped leaf spring wherein the adjustment of said opening causes a corresponding adjustment in .said predetermined temperature level and thus said second predetermined time period. 

1. A device for controlling the ignition and combustion of a burner said device cOmprising a. spark gas means for providing a spark to ignite said burner; b. ignition transformer means for coupling energy to said spark gap means; c. main capacitor means for discharging energy stored therein through said ignition transformer means; d. normally non-conducting thyristor means connected in series with said main capacitor means and said ignition transformer means to form an ignition circuit for providing the discharge path of said main capacitor means through said ignition transformer means; e. first timer means connected in series with said main capacitor means for completing said ignition circuit and rendering said thyristor means conducting after a first predetermined time period; f. flame detector means responsive to a predetermined postignition burner flame for interrupting said ignition circuit and rendering said thyristor means non-conducting; and g. safety switch means connected in series with said flame detector means and said first timer means for timing a second predetermined time period after said first predetermined time period and turning off said burner after said second predetermined time period, wherein said flame detector means prevents said safety switch means from completing said second time period upon the detection of a flame whereby said safety switch means turns off said burner only when said burner is in an operating state and no flame is detected.
 2. The device as set forth in claim 1 further including means for purging said burner wherein said burner is prepurged at the beginning of said first predetermined time period.
 3. The device as set forth in claim 1 wherein said safety switch means includes a heater and a heat sensitive switch wherein the detection of a flame by said flame detector means cuts the current through said heater thus preventing it from operating said heat sensitive switch.
 4. The device as set forth in claim 1 wherein said first timer means includes a thermal switch comprising a heater and a normally open switch.
 5. The device as set forth in claim 1 wherein said first timer means comprises a. first Schmitt trigger means in series with said flame detector means; b. relay means having a contact in series with said main capacitor means; c. second Schmitt trigger means for energizing said relay means; and d. timing capacitor means for switching said second Schmitt trigger means after said first predetermined time period, thus energizing said relay.
 6. The device as set forth in claim 4 wherein said safety switch means comprises a. a microswitch; b. a first leaf spring for switching said microswitch; c. a stationary bracket; d. a U shaped leaf spring mounted on said bracket, the open end of said U being adjustable; e. an arm mounted on one arm of said U shaped leaf spring; and f. a J shaped bimetal piece mounted on said arm and positioned to hold said first leaf spring in a position wherein said microswitch is in a first state whereby upon the heating of said bimetal piece to a predetermined level, said bimetal piece moves to a position to release said first leaf spring thus switching said microswitch to a second state, wherein the time required to reach said predetermined level is said second time period.
 7. The device as set forth in claim 6 said safety switch means further including means for adjusting the opening of said U shaped leaf spring wherein the adjustment of said opening causes a corresponding adjustment in said predetermined temperature level and thus said second predetermined time period. 